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Can You Overcharge Rechargeable Batteries?

Recent innovations in the rechargeable battery world have made the regular use of rechargeable batteries more attractive and practical. With all of the rechargeable options available, Are there suggested techniques when recharging those batteries? Can they be overcharged?

Rechargeable batteries can be overcharged, and they can overheat or die from improper charging. However, they have to be neglected or the wrong battery charger has to be used to make this happen.

It’s a better world with rechargeable batteries like Panasonic’s Eneloop Pro Rechargeable Batteries (on Amazon). Even as we collect and use so many handy electronic devices, keeping their often toxic energy sources out of landfills is of benefit to all. Keeping their useful lives maximally energetic and long is also beneficial to our bottom line, as rechargeables will likely remain slightly higher in price compared to their one-shot predecessors.

How Rechargeable Batteries Work And What They’re Made Of

All batteries are made up of two different materials, separated by a barrier, (however, as we’ve stated before, not all chargers work with all batteries so keep that in mind). One of these materials, called the anode, likes to release electrons. Electrons are the negatively charged particles that, when flowing along a conductor, such as a wire or an electrical circuit, make electricity.

The other material, called the cathode, likes to collect electrons. Together these two materials create a useable electrochemical potential, and the right quantity and quality of the two materials can create a long-lived and consistent electrical current. Eventually, all batteries lose their capacity to shed and collect electrons; this point is called the battery’s terminal voltage.

Rechargeable batteries can be made up of many different kinds of materials. A classic example of a rechargeable battery is a car battery, which use a combination of lead plates and a strong acid. For consumer electronics, rechargeables may be made of nickel and cadmium (NiCad), nickel and a metal hydride (NiMh), or more recently, ionized lithium (lithium-ion).

Rechargeable batteries, because of the chemical structure of the materials used in their construction, have the ability to “switch gears,” and return most of the electrons absorbed by the cathodic material back to the anode. This is accomplished by placing an electrical current going the opposite direction across the battery terminals and is the main function of a battery charger.

How Overcharging Hurts Rechargeable Batteries

Each rechargeable battery type can utilize this recharging power only when delivered at the precise voltage and amperage levels. If the wrong amount of power is used when recharging a battery, by connecting it to the wrong charger for the rechargeable battery type, for example, the battery can become overcharged.

Each type of rechargeable battery reacts differently to being overcharged. NiCad and NiMh batteries can each handle a little overcharging, and dissipate the excess energy as heat. Lead-acid car and marine batteries are designed to be continuously charged, and so they can tolerate a moderate amount of overcharging too. However, lithium-ion rechargeable batteries can explode when overcharged.

How to Recharge Batteries Properly

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Battery hygiene describes the proper behavior regarding disposable and rechargeable batteries. Like with any electrical device, rechargeable batteries, while easy to use and understand, still require a little management on the human end in order to function properly and maximize their useful working life which is just one reason why we’ve written multiple guides including one on the number of times you can actually charge a rechargeable battery.

Use the Charger That Came with the Batteries

In general, it’s best to use the charger that came with the batteries. This assures that the correct power levels get delivered to the rechargeable batteries. Some chargers are suitable for more than one battery type, but this will be clearly indicated on the charger. Again, if the charger doesn’t indicate this, or if you can’t tell what your batteries are made of, use the original charger.

NiMh and NiCad rechargeable batteries are often packaged with inexpensive chargers that do not have a trickle-charge phase and do not automatically turn off. This is because these types of batteries can tolerate being overcharged, simply heat up, and will not leak material at the chargers’ power levels.

Don’t Leave Your NiMH, NiCad, or Lithium-ion Batteries in the Charger

Leaving your NiMh and NiCad batteries plugged into a “dumb” charger can eventually become too much of a good thing. The heat built up within the battery by excessive charging will begin to affect the internal shape of the battery and ultimately reduce its electrochemical potential. Most sources say that charging these kinds of batteries for more than 24 hours is excessive.

Why Lithium-ion Batteries Are Worse To Leave Plugged In

For lithium-ion rechargeable batteries (more on this in our guide), the issue is more profound. Overcharging can lead to leakage of material and ultimately explosion, as the excess current eventually damages the material separating the anode and cathode.

For this reason, all good lithium-ion chargers have a trickle-charging feature, which, when detecting that the connected rechargeable batteries have reached their peak charge, stops the outflow of current. As the batteries naturally release a small amount of charge, the charger then resumes charging until the peak is again quickly reached.

For lithium-ion batteries, while the trickle-charging feature eliminates the risk of dangerous overcharging, leaving these batteries connected to a charger for too long can degrade their useful life. For all kinds of rechargeable batteries, controlling the time they’re plugged in, perhaps by using an electrical timer, assures you’re getting the most from your batteries.

How to Store NiCad, NiMh, and Lithium-ion Rechargeable Batteries

For NiCad batteries, the optimal temperature is between −4° F to 113° F. For long term storage, it’s best to discharge them completely, around 0% to 40% of their maximum charge. For very long periods of dormancy, NiCads form crystals that will internal short out the battery, making them less efficient and powerful, so it’s best to charge them at least once per year.

NiMh batteries are best stored between −4° F to 95° F, and can remain safely in longer-term storage in a charged or uncharged state. If left for a long period of time, a year or more, when recharging the NiMh battery may not reach its maximum charge right away. This can be remedied by charging and discharging the battery several times.

When storing, lithium-ion batteries should not generally be allowed to discharge to level less than 2 volts. At this point, the anode of the battery can begin to dissolve, reducing and ultimately eliminating the electrical capacity of the battery. Lithium-ion batteries are best stored at around 50° F, with their terminals covered and protected.